There are various conventional fan revolution control methods for controlling the fan revolution speed of a cooling fan adapted to be rotated by a fan motor, wherein the control of the fan revolution speed is executed by controlling the pump-motor system that serves to drive the aforementioned fan motor by means of hydraulic oil supplied from an engine-driven fan pump. Some of such control methods call for detecting a temperature of a fluid to be cooled (hereinafter referred to as cooling target fluid) by means of a temperature sensor and, based on the detected temperature of the cooling target fluid, determining a target fan revolution speed of the cooling fan that serves to cool the cooling target fluid (refer to Japanese Patent Publication No. 3295650 as an example).
The method of determining the target fan revolution speed in the fan revolution control method described in Japanese Patent Publication No. 3295650 has a distinctive feature: once a target fan revolution speed is determined, a pump capacity command electric current value, which is a control signal corresponding to a target fan revolution speed or a maximum revolution speed, is step input into an electro-hydraulic transducing valve as shown in the middle graph in FIG. 7 when an engine is started up. The aforementioned electro-hydraulic transducing valve may be an electromagnetic proportional control valve or the like and serves to control a capacity changing means of a fan pump.
A target fan revolution speed command value can be set in the range from a minimum fan revolution speed to a maximum revolution speed. In the case of the method described in Japanese Patent Publication No. 3295650, the target fan revolution speed is determined based on a target temperature, and a command signal is output to set the target fan revolution speed at a value within a range between the minimum fan revolution speed, e.g. 300 rpm, and the maximum revolution speed, e.g. 873 rpm. If the command value for the target fan revolution speed is set at, for example, 873 rpm, in accordance with the detected temperature data from the temperature sensor, the revolution speed instantly increases from 300 rpm (the minimum fan revolution speed) to 873 rpm in a step-like manner when the engine is started up for the first time. An example of a case of a step input of a pump capacity command electric current value is shown in the middle graph in FIG. 7.
As the conventional method described above calls for step input of a control signal that corresponds to a target fan revolution speed or a maximum revolution speed at the start-up of an engine, a great load is applied to a pump-motor system, which comprises a system from a fan pump to a fan motor, and causes a peak pressure or pressure hunting, resulting in the possibility of damage to the pump-motor system.
As is evident from measurement data regarding pressures of the fan pump and the fan motor shown in the upper graph in FIG. 7, a peak pressure may be generated in the pump discharge pressure of the fan pump (or the motor inlet pressure), or a great difference may arise between the motor inlet pressure and the motor outlet pressure of the fan motor and cause pressure hunting of the motor outlet pressure. Such a peak pressure or pressure hunting may cause damage to the pump-motor system, which comprises a system from the fan pump through a piping to the fan motor.
In order to solve the above problems, an object of the invention is to provide a fan revolution control method for controlling the fan revolution speed of a cooling fan by controlling the pump-motor system that serves to drive a fan motor by means of hydraulic fluid supplied from a fan pump, wherein the fan revolution control method enables prevention of a peak pressure or pressure hunting, either of which may otherwise cause damage to the pump-motor system.